1. Field Of The Invention
This invention relates to supporting a fabricated piece for purposes of treating said fabricated piece and in particular to a method for supporting a thermoplastic piece for annealing.
2. Background Of The Art
Methods for annealing plastics are commonly known in the art. In general, plastics are annealed to remove residual stresses and strains, to stabilize the shape of a plastic piece, and to reduce or eliminate defects in the piece. After forming a plastic piece by means of injection molding or extrusion, residual flow induced stresses may remain in the piece, as well as stresses from the differential density, shrinkage, and the viscoelastic behavior of the plastic during nonuniform cooling. Stresses are usually induced by quickly cooling a polymer from above its glass transition temperature. Molded thermoplastics containing residual stresses are more likely to develop stress crazing and stress cracking. To alleviate the effects of internal stresses, plastic pieces are usually annealed. Annealing typically comprises reheating the plastic to above its glass transition temperature where chain mobility is greatest, and then slowly and gradually cooling the plastic to avoid reintroducing stresses. Plastic which is heated above its glass transition temperature usually relieves internal stresses by changes in density and specific volume.
Moreover, some degree of crystallization of the plastic may be achieved by heating it to a temperature above its glass transition temperature. Consequently, a freely held piece will often undergo distortion and shrinkage. Presently used equipment for annealing plastic usually comprises either liquid baths or hot air. Liquid bath methods using water, oil or waxes, usually make some provision for recycling the liquid to prevent temperature build-up or hot spots. Air flow methods usually employ circulating-air or air flow ovens. Other methods such as using convection ovens infrared ovens, and dielectric (e.g. microwave) ovens are also employed. Generally such methods require means such as racks or clamps to hold or suspend the plastic piece in the annealing oven.
Plastic pieces used in surgical procedures often have complicated structures and must operate in conditions where close tolerances are required. Even small distortion or shrinkage in plastic pieces such as surgical fasteners is undesirable. Racks or simple clamping means are not effective for eliminating distortion or shrinkage, hence the use of fixturing, which involves supporting the plastic piece along substantially its entire external surface, usually by placing the plastic piece in a device closely resembling the mold from which the piece was originally fabricated. Pieces which have fine surface detail require expensive fixtures to match and support the detailed configuration of the piece.
Annealing is but one example of various processes which require a work piece to be supported in this manner. Other treating processes which require access to the surface of the work piece, such as those entailing chemical reactions, diffusion, coating, etc., often cannot be efficiently performed with the customary fixturing apparatus and methods.
To overcome problems such as those described above, the present invention provides a novel and inexpensive method to support a work piece for undergoing treatment processes, such method providing support over substantially the entire surface of the work piece while allowing access to the surface for various treatment and conditioning processes. The invention further provides a unique method for supporting a work piece having at least one hollow or cut-out section either internally or externally so as to maintain an equilibrium between internal and external pressure forces applied to the work piece. The invention also provides a surgical fastener constructed of a material treated in accordance with the method disclosed herein.